The Topographic Feature of the Sea-bed in South China Sea
The seabed in South China Sea boasts its rich varieties of its topographic feature including the vast continental shelf, the continental slope and deep sea basin. The general topographical feature of South China Sea is characterized by its higher northwest part, lower southeast part, trapezoidally descending from peripheral part to the central part, continental shelf at the marginal part of the lozenge basin, the trapezoidally descending continental slope at the outer side of the continental shelf. At the central part of the South China Sea¡¯s sea basin lies a flat deep sea basin. The sea bed ridge alternates with the lower part among which troughs and submarine trenches rise and islands as well as coral reefs are widely distributed.
The South China Sea continental shelf, being the gently extending part of Asian continental shelf, mainly lies in the north, west and south parts of the sea area. Because of its gentle topography, the peripheral water depth to the east of Pearl River mouth takes on an average depth of less than 200 meters, while to the west of the river mouth, the peripheral water depth increased with the widening of the water course with a general water depth of over 200 meters and the deepest of 379 meters. Broadly speaking, there are broader continental shelves in the northwest part and southwest part of the sea area, and at the same time, the relatively narrower part in the east.
Being the natural extension of the continental shelf, South China Sea¡¯s continental shelf lies on the peripheral part of the continental shelf, covering a rough area of 1.2 million kilometers which represents 49% of the total, it is the most broadly distributed topographical unit in the South China Sea area. The water depth is between 150 meters to 3,600 meters. There exists a terminal depth of 3,200~3,500 meters in the north east part and north part of the basin, while the other parts sees a terminal depth of 3,800~4,000 meters. Trapezoidally descending is one of the main characteristics of this area. According to the study, five sections of steps arising from South China Sea continental shelf as follows:
Affected by the controlling of fragmentation structure in north-east, north-north, east-west and north-west directions, deep-sea troughs, sea-bed plateaus, slope terraces, sea-bed steep slope and sea-bed canyons are developed in this area.
South China Sea¡¯s continental slope consists of the following
Abyssal plain, covering a total area of 400 thousand square meters, is located east to the central part of South China Sea. Encompassed by the continental slope or island slope, Bashi Strait boasts a cannel of 2000-meter-deep connecting with the Philippines. Stretching in the direction of northeast-southwest direction, Abyssal plain of South China Sea runs up to 1500 kilometers with the broadest area of 820 kilometers. The plain stretches towards southeast part from the northwest with a 3400-meter-depth in the north part while 4200 meters in the south, quite a few parts exceeds 4400 meters in depth. The plain¡¯s average ratio of slope is 1¡ë to 1.3¡ë, but the ratio of slope in the north central part of the basin is only 0.3¡ë to 0.4¡ë.
There distributed isolated submarine mountains up to 3400~3900 meters high, which is composed of all the isolated peaks dotted in the basin area. There are 27 sea mounts over 1000 meters high as well as more than 20 sea knolls with the heights ranging from 400 meters to 1000 meters on the abyssal plain.
Respectively on the northeast end and southwest end of the abyssal plain lie two deep-water valley floors. At the valley entrance, as at the ends of all submarine valleys, are piled with different abyssal caves. Some of them have been erected to be northeast-toward small scale ridges.
The abyssal plain of South China Sea, in terms of its topographical structure, is the part forming from the deep layer of basalt¡¯s compensating upwelling caused by the tearing occurred at the rim of Asian continent. There still exists a series of volcano erupting activities along the grand northeast crevice in the central and east part of the plain.